351
Airborne Water Vapor Differential Absorption Lidar
14.3.2.2 Transmitter
The Ti:Sa laser is designed to operate in a double-pulse mode (separated by 400 μs) at
5 Hz, with an output pulse energy of 150 mJ and a pulse duration of 30 ns (FWHM).
It is pumped by a frequency-doubled Nd:YAG laser and injection seeded with a
single-mode LD (Barnes and Barnes 1993; Barnes et al. 1993a,b). The Ti:Sa cavity
is an unstable resonator (as shown in Figure 14.9) consisting of two 18-mm-long
Brewster-cut Ti:Sa rods. Other resonator components include an output coupler
(graded reflective mirror), a high reflective end mirror, a four-plate birefringent filter,
and a hollow retro-reflector. The total cavity length is 1.5 m. The spectral linewidth
of the unseeded Ti:Sa laser is 1 nm, and the tuning range includes the required 813–
819 nm wavelength region. Fine linewidth and wavelength control of the Ti:Sa laser
is achieved by using the single-mode LD as an injection seeder. The LD has 100-mW
output power and is injected through the high reflective end mirror. The spectral
linewidth of the seeded Ti:Sa laser is less than the required 1.0 pm. Greater than 99%
spectral purity is ensured also by injection seeding, which is measured using the
absorption-to-transmission ratio of laser pulse through a 200-m-path-length water
vapor–filled cell. The central wavelength of the Ti:Sa laser is determined by the LD
seeder, the wavelength of which is locked onto a selected water absorption line. A
fraction of the frequency-modulated light of the seeder passed through a multipass
Onboard processing,
real-time color display of
water vapor, aerosol, and cloud profiles,
color printer
Stratosphere
Upper troposphere
Troposphere
Zenith
telescope
Nadir
telescope
Laser
temperature
control
Lamp driver
unit
λ off
λ on
400 µs
Computer
control
Data
recorder
Power
distribution
Nd:YAG
pumped laser
Ti:Sapphire
laser
Detector and signal
processor packages
λ off
λ on
400 µs
FIGURE 14.8 LASE system diagram configured inside the DC-8 aircraft.
Airborne Water Vapor Differential Absorption Lidar
14.3.2.2 Transmitter
The Ti:Sa laser is designed to operate in a double-pulse mode (separated by 400 μs) at
5 Hz, with an output pulse energy of 150 mJ and a pulse duration of 30 ns (FWHM).
It is pumped by a frequency-doubled Nd:YAG laser and injection seeded with a
single-mode LD (Barnes and Barnes 1993; Barnes et al. 1993a,b). The Ti:Sa cavity
is an unstable resonator (as shown in Figure 14.9) consisting of two 18-mm-long
Brewster-cut Ti:Sa rods. Other resonator components include an output coupler
(graded reflective mirror), a high reflective end mirror, a four-plate birefringent filter,
and a hollow retro-reflector. The total cavity length is 1.5 m. The spectral linewidth
of the unseeded Ti:Sa laser is 1 nm, and the tuning range includes the required 813–
819 nm wavelength region. Fine linewidth and wavelength control of the Ti:Sa laser
is achieved by using the single-mode LD as an injection seeder. The LD has 100-mW
output power and is injected through the high reflective end mirror. The spectral
linewidth of the seeded Ti:Sa laser is less than the required 1.0 pm. Greater than 99%
spectral purity is ensured also by injection seeding, which is measured using the
absorption-to-transmission ratio of laser pulse through a 200-m-path-length water
vapor–filled cell. The central wavelength of the Ti:Sa laser is determined by the LD
seeder, the wavelength of which is locked onto a selected water absorption line. A
fraction of the frequency-modulated light of the seeder passed through a multipass
Onboard processing,
real-time color display of
water vapor, aerosol, and cloud profiles,
color printer
Stratosphere
Upper troposphere
Troposphere
Zenith
telescope
Nadir
telescope
Laser
temperature
control
Lamp driver
unit
λ off
λ on
400 µs
Computer
control
Data
recorder
Power
distribution
Nd:YAG
pumped laser
Ti:Sapphire
laser
Detector and signal
processor packages
λ off
λ on
400 µs
FIGURE 14.8 LASE system diagram configured inside the DC-8 aircraft.